This invention relates generally to automatic washing appliances such as dishwashers, particularly to dishwashers employing high wattage heaters for heating the dishwashing liquid during certain wash and rinse cycles. More specifically, this invention relates to an improved control system which prevents further cyclical operation of the appliance in the event of a malfunction in which the level sensing means does not reset following a drain cycle during which liquid is removed from the tub. This provides additional protection against undesirable overheating of the liquid, the heater, or the tub which could occur in the event the heater is energized with insufficient liquid in the tub.
Washing appliances employing heater means for heating the washing liquid and employing float-actuated switch means to enable energization of the heater means when the liquid in the chamber exceeds a predetermined level and to prevent energization when the liquid level is below a predetermined level are well known in the art. U.S. Pat. No. 3,846,615-Athey et al and U.S. Pat. No. 3,083,717-Bear are illustrous of such machines.
U.S. Pat. No. 4,068,114-Johnson et al discloses a dishwasher in which a mechanical float is used to provide positive fill control and heater protection. In Johnson et al, an electrically operated solenoid valve initiates the fill operation and a float-actuated mechanical valve terminates fill when the water reaches a preselected maximum level. Timer control of the solenoid valve provides a redundant fill control. After a predetermined period of time has elapsed, the solenoid valve is closed and terminates fill. This prevents overfill in the event of a failure of the mechanical valve. In addition to closing the mechanical fill valve, the float also controls a switch to prevent energization of the heater disposed in the wash chamber when the level of liquid in the wash chamber either fails to reach or drops below a preselected level.
While the arrangements disclosed in this prior art perform their desired protective functions so long as the float mechanism operates properly, little provision is made for a malfunction of the float means. The float mechanism typically is exposed to the interior of the wash chamber. Thus, it is possible for food particles, small bone fragments, toothpicks, or similar items to become lodged in the mechanism so as to interfere with proper operation of the float mechanism. A redundant fill control, as taught by Johnson et al, eventually turns off the water even though the float becomes stuck in the low level or empty position. However, it does not address the problem of the float becoming stuck in the high or full position. When stuck in the full position the float means would prevent liquid fill while enabling heater energization during subsequent cycles. In such an instance the heater means could be energized with insufficient liquid in the wash chamber, possibly resulting in heat damage to the heater or the tub. Even in those instances when a thermostat is provided to control the liquid temperature, its response in the absence of liquid is may be too slow to adequately protect the heater and tub. Thus, it is desirable to provide a control circuit for a washing appliance such as a dishwasher which provides protection against the float mechanism becoming stuck in the high or full position.
It is therefore an object of the present invention to provide a washing appliance control system which prevents continued operation after a drain cycle if the liquid level sensor indicates higher than a predetermined level of liquid in the machine.
It is a further object of the present invention to provide a control system for a washing appliance which automatically determines whether the level sensing means has reset by the end of a drain cycle and prevents the initiation of subsequent operating cycles if the level sensing means has not reset.
It is a further object of the present invention to provide a control system for a washing appliance which includes a timer motor for controlling cyclical operation of the appliance and which incorporates protective switch means to automatically interrupt energization of the timer motor following a drain cycle in the event the float means remains in the full position at the end of the drain cycle.